I've just built a 29er and am using the 36/22 double and 11-34t 9spd cassette off the 26er. Not a massive problem, but I am (as to be predicted) noticeably using the top half of the cassette a lot more than previously.
Both chainrings are shot, so I'm going to replace (cassette and chain are one ride old - last of the 9spd spares).
To get a similar set-up to the 26er, I'm thinking of dropping the chainrings to 20/32 or 20/34.
Any thoughts on this? Anyone done it? Or what are you running on your 29er?